Referring to FIG. 1, a conventional vertical LED is formed in a sandwich structure which includes an N-type semiconductor layer 1, a light-emitting layer 2 and a P-type semiconductor layer 3. Below the P-type semiconductor layer 3, a mirror layer 4, a buffer layer 5, a binding layer 6, a silicon substrate 7 and a P-type electrode 8 are disposed in sequence. A surface of the N-type semiconductor layer 1 is processed by chemical or physical etching to increase light extraction efficiency. An N-type electrode 9 is disposed on the surface of the N-type semiconductor layer 1.
By applying a voltage between the N-type electrode 9 and the P-type electrode 8, the N-type semiconductor layer 1 provides electrons while the P-type semiconductor layer 3 provides holes. The electrons and the holes are combined in the light-emitting layer 2 to generate energy level hopping to further produce excitation light.
FIG. 2 shows a detailed structure of the conventional buffer layer 5 which consists of two different blocking materials 5A and 5B that are alternately stacked. The blocking materials 5A and 5B are selected from a group consisting of platinum, nickel, titanium, tungsten, copper, chromium, silicon and aluminum that chiefly serve for releasing thermal stress and resisting ion diffusion. The blocking materials 5A and 5B have thermal expansion coefficients between those of the silicon substrate 7 and the epitaxy of the LED and are thus capable of absorbing thermal stress generated by thermal expansion and contraction. Further, as the blocking materials 5A and 5B have stable physical property and high density, they are capable of blocking ion diffusion to prevent the epitaxy structure of the LED from being damaged.
However, the above-mentioned stress releasing structure is prone to deformations caused by extrusion and stretching of stresses when the LED undergoes numerous processes of thermal expansion and contraction. As a result, such a stress releasing structure is likely to be broken and thus forms cracks. Therefore it is apparent that releasing thermal stress merely by using the blocking materials 5A and 5B cannot achieve the thermal stress resisting effect as desired to meet actual requirements.